Servo system for a two-dimensional micro-electromechanical system (MEMS)-based scanner and method therefor

ABSTRACT

A servo control system micro-electromechanical systems (MEMS)-based motion control system (and method therefor), includes a motion generator having an inherent stiffness component.

The present Application is a Continuation Application of U.S. patentapplication Ser. No. 10/411,136 filed on Apr. 11, 2003 now U.S. Pat. No.7,119,511.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a disk drive, and moreparticularly to a servo system for a two-dimensional MEMS-based scanner,and a method for use with the servo system.

2. Description of the Related Art

A micro-electromechanical system (MEMS) can be utilized to generatenanometer-scale motion. While providing nanometer-scale, yet precise,positioning capability (e.g., 5 nm 1-sigma error), it is advantageous tobuild a capability to span micrometer-scale areas (e.g., 100 μm squarerange) in the X-Y plane. The larger span range enhances a scanner'sapplication potential. A key application of such a scanner is in thearea of atomic force microscopy (AFM)-based storage applications, suchas in a system disclosed in Vettiger and G. Binning, “The NanodriveProject,” Scientific American, pp. 47-53, January 2003, and PCTPublication No. WO 03/021127 A2.

In this system, a polymer media for recording information is supportedby a scanner. Unlike a friction-free actuator system, such as the onefound in a disk drive actuator, a MEMS-based scanner is dominated bystrong stiffness-producing flexural elements that provide X-Y freedomfor movement. However, the presence of significant stiffness in theactuator system is shown to produce steady position error with respectto a ramp-reference trajectory in scan mode, and also suboptimal seekmotion to a target track prior to a scan motion.

Thus, a new servo architecture is needed to overcome the effect ofresistance generated by a system of flexural elements (i.e., that areintegral to a MEMS-based scanner) so that two-dimensional seek andtrack-following-scan performances are competitively achieved.

SUMMARY OF THE INVENTION

In view of the foregoing and other problems, drawbacks, anddisadvantages of the conventional methods and structures, an exemplarypurpose of the present invention is to provide a new servo architecture(and method therefor) which overcomes the effect of resistance generatedby a system of flexural elements (i.e., that are integral to aMEMS-based scanner) so that two-dimensional seek andtrack-following-scan performances are achieved.

In a first exemplary aspect of the present invention, a servo controlsystem for a micro-electromechanical systems (MEMS)-based motion controlsystem, includes a motion generator having an inherent stiffnesscomponent.

In a second exemplary aspect of the present invention, a servo controlsystem for a micro-electromechanical systems (MEMS)-based motion controlsystem, includes a scanner having an inherent stiffness, and afeedforward mechanism operatively coupled to the scanner forfeedforwarding a component for counterbalancing the stiffness of thescanner.

In a third exemplary aspect of the present invention, a servo controllerfor controlling movement of a scanner, includes a servo unit forgenerating a first-axis motion and a second-axis motion under atrack-follow-scan mode and a turn-around mode. A scan rate isprogrammable by choosing an appropriate slope for a ramp trajectory forthe servo unit when generating the first-axis motion.

In a fourth exemplary aspect of the present invention, a method ofstorage-centric applications includes performing a two-dimensional seekat a first speed and a first precision, and performing a one-dimensionalscan at a second speed and a second precision. The first speed is higherthan the second speed, and the first precision is less than the secondprecision.

In a fifth exemplary aspect of the present invention, a servo controlsystem for a micro-electromechanical (MEMS)-based motion control system,includes a proportional-integral-derivative (PID) controller including atype-1 system. The controller has a steady position error due to a rampmotion.

In a sixth exemplary aspect of the present invention, a method ofcontrolling a scanner in a microelectromechanical system (MEMS)-basedmotion control device, includes generating a velocity profile for eachX-seek, and managing a stiffness of the scanner.

With the unique and unobvious features of exemplary embodiments of theinvention, numerous exemplary advantages accrue. Indeed, the exemplaryembodiments of the invention described herein develop a servo structurethat augments a conventional control structure, including aproportional-integral-derivative (PID) type, so that the significantstiffness characteristics of a MEMS-based scanner are intelligentlyneutralized through an exemplary feed forward control method.

Thus, the invention provides several examples of a new servoarchitecture which overcomes the effect of resistance generated by asystem of flexural elements (i.e., that are integral to a MEMS-basedscanner) so that two-dimensional seek and track-following-scanperformances are achieved.

The present invention specifically addresses a plurality of functions ofa scanner developed for an AFM-based storage application, including atrack-following-scan and a two-dimensional seek.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other purposes, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1A illustrates elements of an AFM-based storage device 100 with anX-Y scanner 110;

FIG. 1B illustrates details of a probe 120 for use with the AFM-basedstorage device 100;

FIGS. 2A-2C illustrate an optical position sensor 200 employed in a testconfiguration;

FIGS. 3A-3C illustrate seek and scan trajectories and their components;

FIG. 4 illustrates scan mode reference trajectories;

FIG. 5 illustrates an architecture of a servo controller 500;

FIGS. 6A-6C illustrate a transfer function of the scanner along one axisincluding Magnitude (FIG. 6A), Phase (FIG. 6B) and a mass-spring-dampermodel (FIG. 6C);

FIGS. 7A-7B respectively illustrate a measured and a computed open looptransfer function (OLTF);

FIG. 8 illustrates a ramp reference (desired) trajectory for a scan modeand an actual (measured) trajectory;

FIGS. 9A-9C respectively illustrate an effect of plant parameters onposition error for a ramp reference input;

FIG. 10 illustrates a feed forward configuration structure 1000 tominimize the impact of MEMS-stiffness on position error;

FIG. 11 illustrates a ramp reference trajectory and an actual responsewith stiffness compensation servo;

FIG. 12 illustrates a scanner displacement vs. current;

FIGS. 13A-13C illustrate two cases, with and without stiffnesscompensation servo, with position error shown in detail;

FIG. 14 illustrates an alternative configuration to reduce the impact ofstiffness (i.e., feedback mode);

FIGS. 15A-15C illustrate performance of a digital velocity estimator inscan mode;

FIGS. 16A-16E illustrate a single step seek to location-B followed by aPID scan;

FIGS. 17A-17E illustrate cascade steps to location-B followed by a PIDscan;

FIGS. 18A-18E illustrate a velocity servo seek to Location-B followed bya velocity servo scan with stiffness compensation; and

FIG. 19 illustrates a velocity profile and seek/track-follow-scan nodes.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Referring now to the drawings, and more particularly to FIGS. 1A-19,there are shown preferred embodiments of the method and structuresaccording to the present invention.

PREFERRED EMBODIMENT

Among several emerging non-volatile storage technologies, AFM-basedstorage promises to deliver more than 1 terabit/sq. inch areal densityin a compact form factor device.

According to published information, 30-40 nm-sized bit indentations ofsimilar pitch size have been made by a single cantilever-tip assembly ona 50 nm thick polymethylmetacrylate (PMMA) layer (e.g., see P. Vettigeret. el., “The Millipede—More than one thousand tips for future AFM datastorage,” IBM J. Research and Development, Vol. 44, No. 3, pp. 323-340,May 2000).

An integrated view of such a system 100 is shown in FIG. 1A. The system100 includes an X-Y scanner platform 110, an X-Y stationary cantileverarray chip 120 having a plurality of cantilever tip array/probeassemblies (shown in greater detail in FIG. 1B), a storage media 130 onthe X-Y scanner 110, an X-position sensor 140, a Y-position sensor 145,an X-actuator 150, a Y-actuator 155, a stationary base 160, flexuralsupports 170 which provide freedom of motion, and a multiplex-driver180.

As mentioned above, the details of the probe are as exemplarily shown inFIG. 1B. The probe includes a resistive heater 121 coupled between twoparallel beams (unreferenced) having a probe tip coupled thereto(unreferenced). The probe tip is stationary and the storage medium to beread/written to includes pits 122 (representing information in a mannerwell-known in the art) and is movable under the probe. It is noted thatthe pits are formed in a polymer layer 123 formed on a substrate 124 ofthe chip 125. The polymer under the tip (nanometer scale) is sensitiveto the temperature radiation coming from the probe tip. Thus, forwriting to the storage medium (e.g., whenever a pit is desired), acurrent is sent to the probe's heater element which heats the probe, anda pit (indentation) is formed on the polymer.

For reading, the probe is brought close to the polymer, and because ofthe presence of a pit (corresponding to a bit), the amount of heatpulled out of the resistive element is less than the adjacent flat area(e.g., the nonpit area). Thus, the change of resistance can be detected,thereby representing the information therein. Hence, with such a system,high areal density is achieved. Indeed, many thousands of such probesmay be included in an array (e.g., 32×32), thereby allowing reading andwriting simultaneously. Preferably, the probe is fixed and thepolymer/substrate is movable by means of the X-Y scanner system.

Each cantilever-tip/probe assembly 120 is associated with acorresponding data field. (Strategically selected data fields may beassigned to provide X-Y position information for a feedback servo loopas discussed below. Thus, high data rates are achieved by the paralleloperation of large, two-dimensional arrays (e.g., 32×32) ofcantilever-tip (referred to as tip-array) assemblies 120.

Time-multiplexed electronics control the read/write/erase functionsneeded in this storage device by activating the cantilever-tip system120. In the system shown in FIGS. 1A-1C, the tip-array 120 is built andassembled on the stationary chip 125, whereas the storage media (PMMA)130 is deposited on the scanner 110 that is programmed to move in an X-Ydirection relative to the tip-array 120.

Sensing the position of the scanner 110 relative to the tip-array 120allows achieving reliable storage functions. Thermal expansion andmaterial creep over a long period of time can render a nanometer-scalestorage system useless, unless accurate position-sensing and servocontrol functions are embedded in the overall system design. In acommercialized version of the Millipede storage system, a positionsensor technology is embedded within the system.

To validate the present invention, an exemplary optical sensor isemployed that is shown in FIGS. 2A-2C. The sensor system 200 includes anedge sensor probe 210, and was custom built, for example, by MTIInstruments (e.g., see MTI Instruments Inc., Albany, N.Y., USA(www.MTIinstruments.com), employs a light beam transmitted by a lightsource 221 through an optical fiber 222 to shine a light at the movableedge of the MEMS.

The light beam through the optical fiber 222 is deflected by 90 degreesusing a miniature (e.g., 1 mm) prism structure including sets of upperand lower prisms 220, 230, respectively.

In operation, the light beam from a light source 221 that passes over amoveable edge is captured by a prism of the second set of prisms 230(e.g., a lower prism), deflected by another 90 degrees, and istransmitted back to a receiving portion of the sensor electronics. Theamount of light received in proportion to the light sent forms the basisfor the voltage output of the edge sensor 210, and the voltage islinearly correlated to the location of the edge.

More specifically, the amount of light overlapping the prism indicatesthe position of the sensor. If the prism is completely blocked by theX-Y scanner platform, then no signal is returned, whereas if the prismis only 50 percent overlapping the X-Y scanner platform 110, then only50 percent of the light is received, and a signal representing the samecan be output.

Having discussed a way to sense the scanner motion, it is noted thatread/write/erase (referred to as R/W) operations require two broadlydifferent position control capabilities as depicted in FIGS. 3A-3C,including a 2-dimensional random seek and track-follow-scan.

FIGS. 3B and 3C define one of the several possible geometrical layoutsfor data recording where motion along a Cartesian coordinate system 300is shown. The dots 310 in FIG. 3A signify the corner of the boundariesof each data field corresponding to each probe. Also shown is data field320.

As shown in FIG. 3C, a scanner with no control force applied to it(power off) (i.e., a relaxed mode) will initially rest at a “homeposition” denoted by Location-A in FIG. 3C.

Under active operation, for example, when access to a data block 320 isrequired (for a read or a write), the scanner must be moved fromLocation-A to Location-B in two dimensions and preferably in minimumtime. The X-seek is nominally identical for all data blocks, whereas theY-seek is random.

Once Location-B is reached (e.g., through the random seek to a targetdata block), the scanner must come to a stop, and change its velocityvector to move along a track in scan mode (with scan speed) towardsLocation-C, where the beginning of a data block is located. For longdata records, the scanner must be able to reach the end of a track alongthe {+x } axis and then turn around (e.g., turn-around mode) and executea reverse direction scan along the {−} axis, as shown in FIG. 3B.

Thus, from location A-B, the scanner will move at seek speed (e.g., intwo dimensions X, Y), and from location B-C the scanner will move atscan speed (in one dimension X) to scan the track C.

It is noted that, as shown in FIG. 3B, it is desirable to minimize the“overshoot” (e.g., the margin area at location B of the data blockneeded for the scanner to turn around) of the scanner during itsturnaround scan mode, thereby to increase the density of the chip andminimize the amount of wasted polymer space.

The scanner developed for this application has the freedom to move alongX and Y Cartesian coordinates independently. Thus, two distinct positionsensors and two feedback servo loops controlling two electromagneticactuators, schematically shown in FIG. 1, are employed to develop thedisclosed invention. It is noted that, in FIG. 1A, the freedom to movealong X-Y coordinates is in reality provided by a complex system offlexures (details not shown), schematically represented by a single“spring” element for each degree of freedom of motion.

An industry-proven proportional-integral-derivative (PID) positioningservo system is a candidate controller for the MEMS scanner, designedfor the storage application. A characteristic PID controller transferfunction, for example in analog form, is represented by the followingexpression:Controller(Output/Input)=(k _(P) +k _(D) s+k _(I) /s)  (1)where gains k_(P), k_(D), and k_(I), are proportional, derivative andintegral gains, and ‘s’ is the Laplace transform operator. Theparameterization process to compute the gains is well known in thefield. A control system designer would use a dynamic model of thescanner and would derive the gain values to achieve an “optimum” design.

An integrated scanner/servo system is required to perform three criticaltasks.

First, it must move the scanner along the X and Y coordinates to thevicinity of a target track (Location-B in FIG. 3C) in a minimum timeusing a velocity servo in a seek mode. To facilitate a robust andreliable seek to a target track, a desired velocity profile is typicallystored in memory and a velocity servo (in contrast to a position servo)is employed to reach the vicinity of a target track.

Next, the control system must position the scanner on the track centerline (TCL) of a target track using the Y-direction servo with minimumsettle-out time using a position controller of the type shown inequation (1), with k_(I), normally set to 0.

Finally, the Y-servo system enters the track-follow mode with theY-servo having a proportional-integral-derivative type (PID) positioncontroller and the X-servo entering a scan mode desiring a fixed,predetermined scan velocity (by either using a position servo or avelocity servo). This operation is referred to as “track-follow-scanmode” to emphasize that the Y-servo is maintaining the storage mediaalong a TCL as the X-servo persistently maintains a predetermined scanvelocity. Both servos preferably maintain precision againstdisturbances, such as unknown hysteresis effects and vibration.

Scan Mode

FIG. 4 illustrates two reference trajectories to generate X and Y motionunder “track-follow-scan” and “turn-around modes”, where an exemplaryX-scan length of 100 μm is to be achieved in 100 ms (1000 μm/s), and theY position is stepped by 40 nm at the end of a track. The scan rate canbe programmed by choosing an appropriate slope for the ramp trajectoryfor the X-servo.

A complete servo architecture 500 to achieve this operation, as well asthe X-Y seek, is shown in FIG. 5. Architecture 500 includes X-servo 510x and a Y-servo 510 y.

It is noted that, for completely decoupled dynamics of a scanner alongthe X and Y coordinates, the servo system 500 can be selected to haveidentical building blocks, but different controllers (position vs.velocity) may be switched in and out of the servo loop at various phasesof the scanner motion.

The position information is generated by the optical edge sensors(unreferenced in FIG. 5 and similar to those shown in FIGS. 2A-2C) andconverted to a stream of digital numbers (at 5 kHz in this example) byan analog-to-digital converter (ADC) 511 a, 511 y.

A digital controller for each axis includes a position controller block512 x, 512 y, velocity estimator block 513 x, 513 y, velocity controllerblock 514 x, 514 y, reference trajectory block 515 x, 515 y, and a postfilter bank 516 x, 516 y.

Under the supervision of a microprocessor, the functions provided by theblocks are activated appropriately. The computed control output indigital form is sent to a digital-to-analog convertor (DAC) 517 x, 517 yat a rate equal to, or different from, the input sampling rate. Theanalog signal generated by the DAC drives a current amplifier 518 x, 518y, which in turn respectively energizes the actuator 150, 155 of thescanner.

Scanner parameters, such as equivalent mass, spring stiffness, actuatorforce constants, etc., can be different for each X and Y motion, andsome parameters can drift with time and temperature. The block diagramof FIG. 5 can be further enhanced to include calibration functions andother critical or auxiliary operations which are not the subject of thepresent invention, but may be needed to make the servo control effectiveunder different operating conditions.

FIGS. 6A-6B show the measured transfer function magnitude and phase ofthe scanner system corresponding to X-directional excitation, while theY-direction actuator is held inactive (i.e., no Y-drive current).

A second order equivalent model is shown in FIG. 6C, in which thedisplacement x corresponding to an input force F is resisted by a simplespring-mass-damper-like system below a frequency range of 2.5 kHz. Theequivalent spring stiffness k and mass m determine the fundamentalresonance frequency (200 Hz in this case). The damping constant cdetermines the quality factor Q of the fundamental resonance mode. Theexplicit presence of the stiffness term “k” is a key challenge inextracting optimum performance from a commercial product and is asubject of this invention. Beyond 2.5 kHz higher frequency resonancemodes start to contribute to motion along the X axis. In the exemplaryscanner, 3.0 kHz and 5.5 kHz modes are observable. Similar frequencycharacteristics along the Y-axis were observed in this scanner design bythe present inventors.

Thus, the simple schematic of FIG. 6C shows that the system will behavevery well as a simple spring mass system up to about 2.5 KHz.

To enhance nanoscale mechanics, the post filter bank 516 x, 516 y (shownin FIG. 5) can be configured to function as a notch or low-pass filterwith relevant high frequency modes.

Thus, again FIGS. 6A-6C shows that the system of the invention can beviewed as a simple spring-mass system.

FIGS. 7A-7B show an open loop transfer function (OLTF) corresponding toa digital equivalent of a PID controller. The track-follow servo for theY-axis would use a PID digital controller with very similar propertiesto those shown in FIGS. 7A-7B. The computed and measured OLTF agree wellsince the MEMS system has a friction-free motion capability (with mildhysteresis as discussed later).

However, the freedom from friction-induced performance degradation isnow replaced by an explicit “stiffness” term in the plant (i.e., scannersystem) dynamics. As the MEMS-based scanner should achieve precise scanand optimum seek capability, it is important to evaluate its performancecharacteristics in the presence of a strong stiffness term.

Thus, using a reasonably well-known controller (e.g., PID-likecontroller), the flexure-based structure can be measured and modeled tofit to these curves. Thus, FIGS. 7A-7C show servo compensation beingemployed in addition to the basic spring mass system characteristicsshown in FIGS. 6A-6C.

If one wishes to perform a scan using the position controller, then FIG.8 shows a comparison between a desired scan trajectory and a measuredscan trajectory under PID position control. Such a ramp (positive scan)in FIG. 8 is somewhat similar to what is shown in FIG. 4, but isimplemented using a position controller. The scan rate of (R=) 500 μm/sproduces a steady position error of 250 nm. Since the actual position isknown through direct measurement and the actual velocity is still equalto the desired value, the position error with respect to the referenceramp may not be detrimental under certain Read/Write conditions.

However, when the scanner trajectory is to be flexibly programmed usingan arbitrary reference trajectory, position error becomes an impediment,and it distorts the actual trajectory from the desired one. The positionerror “e” under a ramp trajectory represented by x=Rt, where “t” is thetime, can be shown as:e=R k_(stiffness) /k _(I)  (2)

For a stiffness-free system, for example the case of a bearing-supportedmass, the stiffness contribution is minimum, and the error term “e” isnear zero.

For a MEMS with significant stiffness, equation (2) demonstrates thatthe error grows linearly with stiffness. Especially in cases where thescan rate “R” is increased for certain error recovery or retryoperations during a R/W, the position error “e” can grow as well. Theerror term can nevertheless be reduced by increasing the integral gainterm “k_(I)”, but this method has limitations arising from control andstability considerations. Thus, an alternative method to minimize theerror “e” is desirable.

In characterizing structural properties of a control system, the OLTFsare classified as type-0, type-1, type-2 . . . systems (e.g., see S.Gupta and L. Hasdorff, Fundamentals of Automatic Control, John Wiley &Sons, Inc., p.86, 1970.), where the type order is determined by thepower of the free standing denominator variable “s” of the OLTF. Thus,the term s¹ would indicate a Type-1 system.

FIGS. 9A-9C summarize three cases of a mass (m)-spring (k)-damper (c)system.

In the case of FIG. 9A, the plant is free to move along the x directionunder an excitation force F with no resistance. The plant transferfunction (TF) (e.g., LaPlace transform) thus has a “s²” term in itsdenominator. Under a PD or a PID feedback control, the control systembecomes a type-2 or type-3 respectively. (Note that an integrator in aPID control introduces an extra term “s”, whereas a PD control willnot.) The steady state error due to ramp reference input for a system oftype-2 or higher can be proved to be null, as schematically shown forcases in FIGS. 9A-9C.

In the case of FIG. 9B, if there is only damping and no stiffness, forexample the mass is immersed in a viscous liquid, then the new plant“s(m s+c)” has a single power for the free standing “s”. Under PD or PIDcontrol, the OLTF becomes either type-1 or type-2. It can be shown that,for a ramp input with a PD controller, there will be a steady-stateposition error, but with PID the error is null.

In the case of FIG. 9C which is more realistic and is the case for aMEMS device, the OLTF with PD or PID will be either type-0 or type-1. Itwill be illustrative to set up the OLTF for the PD and PID cases asfollows:

$\begin{matrix}{{{with}\mspace{14mu} a\mspace{14mu}{PD}\mspace{14mu}{controller}\mspace{14mu}{OLTF}} = {\left( {k_{P} + {k_{D}s}} \right)/\left( {{ms}^{2} + {cs} + k} \right)}} & (3) \\\begin{matrix}{{{with}\mspace{14mu}{PID}\mspace{14mu}{controller}\mspace{14mu}{OLTF}} = {\left( {k_{P} + {k_{D}s} + {k_{I}/s}} \right)/\left( {{ms}^{2} + {cs} + k} \right)}} \\{= {\left( {{k_{D}s^{2}} + {k_{P}s} + k_{I}} \right)/\left\lbrack {s\left( {{ms}^{2} + {cs} + k} \right)} \right\rbrack}}\end{matrix} & (4)\end{matrix}$It is observed that the power of the free standing “s” variable in thedenominator of the OLTF is either 0 or 1, respectively. It can be shownthat the corresponding error is either infinity or finite (equation 2).Experimental evidence, shown in FIG. 8, confirms that the steady stateposition error is finite for a ramp reference input with a PIDcontroller. With a simpler PD controller the error is unbounded andgrows with the amplitude of the ramp input.

The basic mechanism creating an error “e” is that, as ramp referencedisplacement increases, the actual stiffness of the spring creates anincreasing resistance to motion. Thus, a fixed gain term in a PDcontroller (equation (3)) can only produce a proportionally increasingdrive force by growing the position error term with time at best.

In the case of a PID controller, the integrator can produce acontinuously increasing drive force by means of a bias error in theposition represented by equation (2).

To minimize the error challenge, one approach is to introduce a doubleintegral in the controller. However, this method has stabilityimplications, since each integral introduces a 90-degree lag in thephase of the OLTF.

The present invention solves the stiffness-based resistance to motion byproviding a counter balancing force through electronic means. If theactual or desired position of the scanner is known, then anelectronically-generated force can be applied through the actuator toeliminate the resistance to motion.

When this form of counter balancing is augmented with a conventional PIDcontroller, then the steady state position error for a ramp referenceinput is minimized, while preserving the merits of a feedback controlsystem.

Thus, now that it is known from FIGS. 9A-9C how the mechanism works inproducing the steady state position error, since the scanner system hasa stiffness which is measurable, and it is known what is desired when aramp motion is to be performed, such a burden need not be placed solelyon the servo controller.

Instead, FIG. 10 shows an exemplary structure 1000 of a feed forwardconfiguration in which the anticipated stiffness term is canceled by afeed forward element (e.g., through a stiffness term including either ak_(stiffness) unit 1020 for linear stiffnesses, or a look up table 1030for complex stiffnesses).

Thus, in this exemplary embodiment the target term (e.g., the targetreference) can be fed through the stiffness term digitally to theactuator as a current. Indeed, since it is known at each moment adesired position, if the corresponding spring force can be neutralized,then there is typically no restoring force which needs to be applied bythe controller itself. Thus, the exemplary approach of the presentinvention is to feed forward the stiffness term without waiting for thecontroller to build up.

That is, in the structure 1000 of FIG. 10, an input target positionX-reference value (term) is provided to a node (e.g., a summing node)1010, a k_(stiffness) unit 1020 (for linear stiffness; case A in which kwill be a constant term) and a table 1030 (for complex stiffness; case Bin which k is a term having a complex, parabolic etc. type waveform).The node 1010 also receives a scanner position signal X-m from thescanner (having a position sensor) 110. The node 1010 takes thedifference between the target position X-reference and the measuredscanner position.

Based on the difference, the node unit 1010 outputs a position errorsignal (PES) to a servo controller 1040, which is also provided with areference velocity input 1050. An output U of the servo controller isprovided to a digital summing node 1060. The digital summing node 1060also receives inputs U_(b) from k_(stiffness) unit 1020 and table 1030,depending upon the linear stiffness or complex stiffness being present.

The node 1060 provides an output to an amplifier K_(A), which in turnamplifies (integrates) the signal from node 1060 and provides a signalU_(out) to the scanner. The scanner 110 in turn provides the scannerposition X-m signal to the node 1010.

Thus, there are two possible approaches to generating the counterbalancing term.

In Case-A, the stiffness is known to be a linear or mathematicallyrepresentable function. In this case, a compact computationalrepresentation from k_(stiffness) unit 1020 would be sufficient tocompute the required actuator current.

In Case-B, the resistance force is a complex function of position. Inthis case, the look-up table 1030 is constructed employing a calibrationmethod in which the quasi-static current (mA) vs. displacement (μm) datais measured and stored therein.

When FIG. 10 is implemented, the results shown in FIG. 11 are obtained.

That is, FIG. 11 shows the positive effect of using the stiffnesscounterbalancing feed forward method during a ramp motion. Compared tothe case corresponding to FIG. 8, the position error component is almostunobservable. A linear approximation to the stiffness is used to computethe counterbalance force. The stiffness term is derived by performing aquasi-static calibration.

By injecting a steady current in steps of 5 mA from the neutral position(Location-A) of the scanner and observing the corresponding equilibriumposition of the scanner, the necessary stiffness term is derived. Theresult of the calibration is shown in FIG. 12.

On the scale of 20 μm displacement, the displacement plot appears verylinear. However, the forward/return motion due to increasing/decreasingcurrent is not identical. The difference between the forward positionand the return position for the same current is plotted as the “delta,”with its scale on the right side of the plot of FIG. 12. A difference ofabout 50 nm can be expected. Likewise, a finer scale calibration nearthe origin may yield a stiffness that is different from the averagestiffness. Further analysis is needed to choose a method to accuratelyrepresent the stiffness term. It is noted that any non-linearity in theactuator force generation capability is implicitly included in thecomposite representation by the plot in FIG. 12.

Thus, the inventors found that, by going from a forward direction andthen going in the backward direction by increasing the current to 40 mAand then decreasing the current from 40 mA, because silicon substrateshave some inherent relaxation in stiffness, then the correspondingposition may not be exactly the same when the current goes back to theoriginal 30 mA, for example. However, the difference will not besubstantial and will not be outrageously inaccurate. By the same token,it will be useful to use a feedback controller to manage the variations,but the gross stiffness element is addressed by the system's feedforwardscheme of the present invention.

The detailed position error characteristics, including the reference andactual trajectories for both cases (corresponding to FIGS. 8 and 11),are shown in FIGS. 13A-13C. The position error has been reduced from 250nm to 50 nm for a nominal stiffness value. The position error componentcan be easily driven to near zero by updating the stiffness term asfrequently as necessary.

The stiffness counterbalancing effect can also be achieved in a feedbackmode in which the measured position is positively fed back, as shown inFIG. 14. Thus, FIG. 14 shows another way to achieve the same result asthe system of FIG. 10. However, in the case of the system of FIG. 14,the stiffness term of the scanner 110 is made to appear as 0 (null) tothe controller.

Hence, since a position sensor is disposed for providing an absoluteposition (or a position with regard to a neutral position), such astiffness term (which is positive) can be fed through a stiffnesselement 1420 (in a digital processor or the like) to a digital summingnode 1460 to counterbalance. Thus, in this exemplary embodiment, thescanner 110 plus the stiffness term (which is a positive equivalentfeedback force) will counterbalance the negative value of the scanneroutput, thereby resulting in a free (floating) system.

In this method, while making the plant appear like a system withoutstiffness, the PID controller must be redesigned to account for themodified plant characteristics. As noted above, the feedback methodrequires reliable position measurement through out its operation. Anoverestimate of the stiffness can also result in an unstable plant whenthe conventional control is not activated as it is in a positivefeedback configuration.

Furthermore, any noise in the position measurement could translate intoa spurious disturbance component, thus generating an undesirablepositioning error. The feed-forward method, using the reference positionsignal, is thus preferable to the feedback method.

Velocity Estimator

Scan mode and seek mode operations require knowledge of the scannervelocity along each axis. Under a velocity servo mode, an estimate ofthe velocity is repeatedly used to generate the control values. Theposition control servo exploits the velocity estimates to ensure, forexample, that the desired switching conditions from a velocity to asettle-out position servo are met at the end of a Y-seek. It is notedthat the cost of embedding a velocity sensor, in addition to a positionsensor, can be excessive and may usurp the electronic circuit resources.Since the scanner position is sampled at discrete time instantsseparated by a fixed duration (i.e., sampling period), a simple estimateof scanner velocity is the arithmetic difference between adjacentposition values. However, in practice the position-difference methodbecomes corrupted by the measurement noise, and newly developedstatistical estimation methods could be considered (e.g., see R. F.Stengel, Stochastic Optimal Control, John Wiley & Sons, Inc., Chapter 4,1986).

A state variable-based full state estimator (including the velocity) isemployed to obtain the estimates of the scanner along the X and Y axes.The following variables are first defined:

-   -   n=Sampling instant;    -   U(n)=Actuator Current Driver input expressed in DAC Bits;    -   Y(n)=Actuator Position Sensor output expressed in ADC Bits;    -   V(n)=Actuator Velocity in ADC Bits/Sample;    -   X1(n)=Estimated Position in ADC Bits;    -   X2(n)=Estimated Velocity (=V(n)); and    -   X3(n)=Estimated Unknown force in DAC Bits.        By casting the scanner dynamics as a second order system with        two state components X1 and X2, and by augmenting the second        order model with an additional state X3 representing the        unmodeled portion of the force (e.g., see M. Sri-Jayantha and R.        Stengel, “Determination of nonlinear aerodynamic coefficient        using the Estimation-Before-Modeling Method,” Journal of        Aircraft, Vol. 25, no. 9, pp. 796-804, Sept. 1988) acting on the        scanner, a state estimator of the following form can be        formulated:        X1(n)=A1*X1(n−1)+A2*X2(n−1)+A3*X3(n−1)+B1*U(n−1)+G1*Y(n)        X2(n)=A4*X1(n−1)+A5*X2(n−1)+A6*X3(n−1)+B2*U(n−1)+G2*Y(n)        X3(n)=A7*X1(n−1)+A8*X2(n−1)+A9*X3(n−1)+B3*U(n−1)+G3*Y(n)  (5)

where constants [A1 through A9], [B1 B2 B3] and [G1 G2 G3] aredetermined by the parameters of the scanner transfer function (TF) andthe desired filtering characteristics of the estimator. The filteringproperty is broadly governed by the characteristic roots of the dynamicsystem represented by equation (5) above.

FIGS. 15A-15C show the effect of the estimator characteristics under ascan mode (e.g., from location B to location C in FIG. 3C). TheseFigures show that a very sophisticated velocity estimator as anexemplary part of the entire implementation of the invention.

FIG. 15A corresponds to a ramp rate of 5000 nm/10 ms, which is alsoequal to a scan rate of 100 nm/sample at a 5 kHz sampling rate. Thus,FIG. 15A shows the measured and estimated position.

FIG. 15B shows a position difference and an estimated velocity,employing matrix equation (5) with the characteristic root at a 1500 Hzradius. FIG. 15B shows the digital estimator to be very “fast”, meaningthat it does not filter very much. As shown, there are many sharp peaks(“wiggles”) in the waveform during the steady velocity, whereas if thereis a redesign of the filter to slow down or to add more filteringcharacteristics (e.g., to filter better) to the estimator, then theresults will be as shown in FIG. 15C in which the velocity is made“smoother”, and thus much better than that of FIG. 15B.

That is, FIG. 15C shows the same position difference plot compared to aredesigned velocity estimator (e.g., Velocity Estimator 2) having a 1000Hz characteristic root. It can be observed that the estimator has thecapability to filter noise depending on the choice of its characteristicroot as a design parameter. An estimator with a 1000 Hz characteristicroot is used in the subsequent application to optimize X-seek.

Thus, the velocity estimator can be designed optimally to have betterfiltering characteristics.

Seek Mode

The seek mode performance is considered for optimization. In the scannerservo, both X and Y directional seeks are required. The Y-seek helps thescanner to move to a target track (e.g., Location-B in FIG. 3C) withzero terminal velocity since the subsequent motion for a R/W requiresthe scanner to maintain the tip-array along the TCL with zero meanvelocity across the Y-axis.

However, the X-seek demands innovative consideration. It not only needsto optimize a seek criteria (such as minimum time or minimum overshootinto the margins of the storage media), it also has to produce a reversevelocity equal to the scan rate along X before a R/W can begin.

Progressively complex control methods can be devised to enhance X-axisseek control. First, three methods will be described below to enhanceX-axis seek control, and then some experimental results will be shown.

The three methods include:

Method—1). A long step input to Location-B from Location-A is first madeusing a PID-like position servo. Once the destination is reached and aterminal velocity of zero is attained, the PID-like position controllerdriven by a ramp-reference trajectory with feed forward stiffnesscompensation is used. Extra space along the X-axis is needed toaccommodate step input overshoot, as well as a “take-off runway” toaccelerate the scanner from rest position to the desired scan speed;Method—2). A cascade of short steps are generated until Location-B isreached, and the scan phase is initiated, as in the above case. In thiscase, the step input overshoot is decreased, but the seek time is likelyto be increased; andMethod—3). A velocity servo is used to follow a reference velocitytrajectory all the way to Location-B where the direction of motion ischanged under the same velocity servo and the scan mode is initiatedusing the same velocity servo. In this approach, the time to move fromLocation-A to a R/W ready condition is observed to be the least.Moderate overshoot space is still required in cases in which thevelocity vector undergoes a 180-degree change of direction.

FIGS. 16A-16B correspond to Method—1. That is, a single step move toregion B is made, to move there fast with some overshoot and thenhesitate a while before following a ramp.

Method 1 does not take advantage of the k_(stiffness), feedforward tothe k_(stiffness), knowledge of the system, etc., but does havefeedforward during the scan. However, this feed forward is immaterial tothis case since it is focussed on moving from location A to location B.

It can be seen that a 5 μm X-axis motion requires about 3 μm overshootand 1 μm for the “take-off runway” needed for scan mode initialization.Before reaching the desired scan velocity, total time is about 11.5 ms.

FIGS. 16B-16D correspond to position (repeat of FIG. 16A), velocity andcurrent commands, respectively.

FIG. 16E shows a two-dimensional representation (e.g., movement in X andY) with no time scale displayed on it. It is noted that the Y-scale ismuch finer (granular) than the X-scale. The seek operation begins fromrest Location-A and moves to a target track near Location-B, followed bya Y-track follow servo and X-scan servo (e.g., referred to as a“track-follow-scan”). The “border” region covered by Location-B wherethe seek to track-follow-scan transition occurs is critical to the R/Wperformance, as well as an effective use of storage media. The scantracks are separated by a 200 nm track pitch in this exemplary test.

Thus, FIG. 16E shows movement from original location A (e.g., theoriginal rest position) to location B, overshoot at location B,turnaround, activate the scan and begin scanning to location C, stepdown, reverse scan across, step down, then perform a scan, etc.

FIGS. 17A-17E correspond to Method-2. This method recognizes that asingle large step may be excessive. Thus, this method attempts tominimize overshoot, but at the expense of total time required which ismuch higher than that required in Method 1.

Hence, in Method—2, a cascade of mini-step moves reduces the overshootto almost 0 □m with 1 □m still needed for the “take-off runway”, buttotal time rises to 15 ms. More specifically, a plurality ofapproximately 0.5 to 1.0-micron size steps leading to the targetposition of approximately −5000 nm in FIG. 17A. However, in theexemplary embodiment, the steps may be permitted to move to −6000 nm toget ready for the scan and to minimize the transient delay.

FIGS. 17B-17D correspond to position (repeat of FIG. 17A), velocity andcurrent commands, respectively.

FIG. 17E shows the two-dimensional representation of the test resultssimilar to that of FIG. 16E. The extended scan along many tracksseparated by a 70-nm track pitch is demonstrated in this example. Thisconfiguration was studied before the stiffness counterbalance method wasdeveloped.

Without exploiting the knowledge of the scanner stiffness, the presentinventors found that it was difficult to design a velocity controller(at a 5 kHz sampling rate) to encompass desirable seek-settlingcharacteristics. The controller not only should accelerate anddecelerate the scanner mass, but it also should build up a continuouslyincreasing and rapidly leveling (near Location-B) counter force againstthe stiffness resistance. While the overshoot distance is minimized, theseek to scan time is lengthened to 15 ms. This is not a competitivetradeoff between border margins for overshoot vs. seek-scan time.

Thus, Method—1 is faster (11.5 ms) to reach the scan mode, but requiresa large border area, whereas Method—2 uses less “real estate” (border ormargin area) but is slower, requiring 15 ms (e.g., about 3.5 ms morethan Method—1) to activate the scan mode.

FIGS. 18A-8E correspond to Method—3, which was designed to optimize theabove-described methods (e.g., optimize both margin and time).Continuous velocity servo for seek and scan requires only 3 ms seek-scantime, and 0.5 μm border space for regaining the scan velocity. Method—3produces the most competitive results in which both seek time and border(or margin) length is minimized.

Optimizing the transition from seek to track-follow-scan operations asaccomplished by Method—3 uses two innovative steps.

A first step is the generation of a velocity profile for each X-seek.The velocity profile that normally would terminate at null velocity whenthe target distance approaches zero should be constructively modified toextend beyond zero as terminal velocity, and should impart a reversevelocity equal to that of the desired scan rate, and continue tomaintain the scan rate until the end of the track is reached. (At theend of a track, the turn-around occurs. This is achieved by a step moveby a Y-position servo, while the X-scan servo produces the same scanrate in the opposite direction.)

A schematic of the velocity profile and modes of the X-Y controllers areshown in FIG. 19.

A second optimizing step is that of managing the “stiffness” problem.Higher sampling rates facilitate easy design tradeoffs. At samplingrates envisioned to be competitive, the seek controllers are found torequire augmentation. The anticipated force to keep the scanner atequilibrium near Location-B can be computed from the knowledge ofstiffness as discussed above.

Hence, to assist the acceleration (in −ve direction along the X axis) astep change in controller output equal to the equilibrium value isgenerated. The velocity estimator is activated by this control output inaddition to the velocity servo output that attempts to follow thereference velocity profile.

FIGS. 18A-8B show the X-seek and scan performance for a 5 μm move withdifferent vertical scales. Five seek and scan operations are repeated toshow the robustness of the access operation.

FIGS. 18A-8B show the time evolution of position from Location-A toLocation-B along the X axis (−ve).

FIG. 18C shows the estimated velocity. It is observed that the peakvelocity of 1250 nm/sample is achieved in 6 samples (1.2 ms), hardlyenough for the controller to build up against the scanner's stiffness.

FIG. 18D shows the stiffness feed-forward output alone (e.g., arelatively powerful output current), and the velocity controller outputwhen it is added to the stiffness feed forward output. The feedforwardstiffness term allows the controller to adjust its behavior to thetrajectory as exemplarily shown in FIG. 19. It is observed from thisplot that the servo controller output (without the stiffness term) ispositive for the first 3 samples, and negative for the next 7 samples inthis example. The net actuator current is almost always in onedirection, indicating that the deceleration is provided by the stiffnessof the scanner alone. The velocity controller cushions the decelerationlevel by the spring so that the transition to scan-mode is achieved inlimited samples. It has been demonstrated that a conservative seek timeof 10-15 ms can be reduced to 3 ms through the two innovative stepsdemonstrated in the present invention.

The stiffness feed forward component can be optimized further by makingit more complex. By stepping the output level in conjunction with theacceleration/deceleration/scan phases, the move time can be furtherreduced. This is a subject beyond the scope of the present invention.The switching criteria that will be universal for all seek lengths,especially when the X-Y dynamics are coupled, can be difficult toachieve and needs further effort.

FIG. 18E, similarly corresponding to Method—3, illustrates thetwo-dimensional seek performance of the system of Method—3.

Thus, Method—3 implements the velocity trajectory along with thestiffness feedforward, as shown in FIG. 18D. In an exemplary embodiment,Method—3 preferably employs the system of FIG. 10 using Case-A, and inwhich the servo controller 1040 is fed with the reference velocity 1050,whereas Methods 1 and 2 in exemplary embodiments preferably employX-position controller 512 (as opposed to the velocity controller).Obviously, other configurations are possible as would be known by one ofordinary skill in the art taking the present specification as a whole.

With the above-described unique and unobvious exemplary embodiments ofthe present invention, a servo structure is developed that augments aconventional control structure, including aproportional-integral-derivative (PID) type, so that the significantstiffness characteristics of a MEMS-based scanner are intelligentlyneutralized through an exemplary feed forward control method.Additionally, a feedback control method is described in which numerousadvantages accrue.

Thus, as described above, the invention provides several examples of anew servo architecture which overcomes the effect of resistancegenerated by a system of flexural elements (i.e., that are integral to aMEMS-based scanner) so that two dimensional seek andtrack-following-scan performances are achieved.

Further, the present invention addresses a plurality of functions of ascanner developed for a AFM-based storage application, including atrack-following-scan and a two-dimensional seek.

While the invention has been described in terms of several preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Further, it is noted that, Applicant's intent is to encompassequivalents of all claim elements, even if amended later duringprosecution.

1. A servo control system for a micro-electromechanical system (MEMS)-based motion control system, comprising: a motion generator having an inherent stiffness component; means for generating a counter balancing term to said inherent stiffness component; a node coupled to receive an input from said means for generating a counter balancing term; and a servo controller for receiving a position error signal based on a target position first-axis reference signal, and a reference velocity, wherein the stiffness term is fed forward without waiting for the servo controller to build up wherein said means for generating comprises: a look up table for a condition when the stiffness comprises a complex function of position.
 2. A servo control system for a micro-electromechanical system (MEMS)-based motion control system, comprising: a scanner having inherent stiffness; and a feed forward mechanism operatively coupled to said scanner for feedforwarding a component for counterbalancing the stiffness of said scanner; a node coupled to receive an input from said feed-forward mechanism, wherein said feed-forward mechanism comprises one of a linear stiffness unit and a look-up table for storing therein complex stiffnesses, for generating a stiffness component, based on a target position first-axis reference signal, for being input to said node; and a servo controller for receiving a position error signal based on a target position first-axis reference signal, and a reference velocity, wherein the stiffness term is fed forward without waiting for the servo controller to build up, wherein the stiffness of said scanner, when counterbalanced by said feed forward component, minimizes a position error of said scanner due to a ramp motion.
 3. A servo control system for a micro-electromechanical system (MEMS)-based motion control system, comprising: a scanner having inherent stiffness; a feedforward mechanism operatively coupled to said scanner for feedforwarding a component for counterbalancing the stiffness of said scanner; a node coupled to receive an input from said feed-forward element, wherein said feed-forward element comprises one of a linear stiffness unit and a look-up table for storing therein complex stiffnesses, for generating a stiffness component, based on a target position first-axis reference signal, for being input to said node; and a servo controller for receiving a position error signal based on a target position first-axis reference signal, and a reference velocity, wherein the stiffness term is fed forward without waiting for the servo controller to build up, wherein a seek motion control is optimized by initiating a seek with a counterbalancing force that is expected at an end of the seek or beginning of a scan position.
 4. The system of claim 3, wherein a first-axis-velocity profile for the seek completes the seek motion with a desired scan speed.
 5. A servo control system for a micro-electromechanical system (MEMS)-based motion control system, comprising: a motion generator having an inherent stiffness component; a node coupled to receive an input from said motion generator, wherein said motion generator comprises one of a linear stiffness unit and a look-up table for storing therein complex stiffnesses, for generating a stiffness component, based on a target position first-axis reference signal, for being input to said node; a servo controller for receiving a position error signal based on a target position first-axis reference signal, and a reference velocity; and a digital velocity estimator having a known bias allowing an estimate of velocity in a flexure system of the MEMS-based motion control system, wherein the stiffness term is fed forward without waiting for the servo controller to build up.
 6. A servo control system for a micro-electromechanical system (MEMS)-based motion control system, comprising: a motion generator having an inherent stiffness component; a feedback element for canceling the inherent stiffness component; a node coupled to receive an input from said feedback element, wherein said feedback element comprises one of a linear stiffness unit and a look-up table for storing therein complex stiffnesses, for generating a stiffness component, based on a target position first-axis reference signal, for being input to said node; a servo controller for receiving a position error signal based on a target position first-axis reference signal, and a reference velocity; and a scanner comprising the motion generator, wherein the stiffness term is fed forward without waiting for the servo controller to build up and wherein the stiffness term of the scanner is made to appear as null to a controller. 